Control device for vapor discharge vessels



Fb. 19, 1946. P. HUBACHER CONTROL DEVICE FOR VAPOR DISCHARGE VESSELSFiled Sept. 9, 1943 Patented Feb. 19, 1946 CONTROL DEVICE FOR VAPORDISCHARGE VESSELS Paul Hubacher, Zurich, Switzerland, assignor toAktiengesellschaft Brown,

Baden, Switzerland Boveri & (lie,

Application September 9, 1943, Serial No. 501,704 In Switzerland May 29,1942 2 Claims. (01. 315-189) For the transmission of direct currentpower over long distances very high direct voltages are used for reasonsof economy. These voltages are generally much higher than those whichcan be dealt with by single vapor discharge devices with a liquidcathode. It is therefore necessary to connect several such dischargevessels in series. One point of the direct current circuit is connectedto earth and thus has earth potential. Both the adjoining vessels thenbehave in a normal manner as regards their voltage stress relative toearth and their control also presents no particular difiiculty.

Each further discharge vessel connected in series with the first vesselis, however, at a higher potential than'its predecessor. Not only doesthis demand a better insulation to earth but difficulties are alsoencountered as regards the control and auxiliary current circuits. Thepower for the auxiliary apparatus such as vacuum pump, ignition device,excitation and cooling installation, has to be supplied over specialinsulating transformers to the individual stages. Special devices arealso necessary for the transmission of the variable control impulses tothe grids of the discharge vessels operating either as controlledrectifiersor inverters. A mechanical displacement of the ignition pointby means of a regulating and control apparatus which is at earthpotential cannot meet the requirements. Especially when rapid controloperations have to be undertaken such as a sudden change-over fromrectifier to inverter operation in the case of short circuits on thedirect current line, the delays caused by the acceleration of the movingmasses are much too long. A further disadvantage of such a controlsystem is the necessity of having to provide devices for each stage forthe production of control voltages which are a far as possiblerectangular in form. In order to increase the safety of the plant it isdesirable that as much as possible of the entire control apparatusshould be at earth potential. This is the only way by means of whichsensitive parts can be easily supervised and repairs carried out withoutinterrupting the transmission.

The present invention concerns a device for influencing the controlgrids by means of impulses and is characterised by the feature that thecontrol impulses are produced with the shape and phase positionnecessary for control purposes in an apparatus which is at earthpotential and are then supplied in the form of modulated high frequencycurrents to the grid circuits which are at high potential of thedischarge vessel. By tun- I The control grids 4, 4"

ing the high frequency circuits to the frequency of the carrier wave,the disturbing voltage peaks of power line frequency and its harmonicsthat during the commutation process in rectifiers and inverters, arewithheld from the control circuits. The higher the carrier frequencyrelative to the maximum disturbing frequency which oc curs, the moreeasily can the aforementioned advantage be obtained. A short wave isthus preferably selected for the carrier wave, this being a frequencyrange in which the radiation losses can still be readily controlled. Forthe transmission of the control impulses by means of high frequency ahigh frequency cable is used. High frequency insulating transformers,which do not need to have an iron core, are used for coupling the cableto the high. potential grid circuits. The high frequency impulses arethen rectified and applied to the control grids as direct currentimpulses in the original form.

A constructional example of the invention is illustrateddiagrammatically in the accompanying drawing.

In the arrangement shown are two rectifier groups i, I connected inseries, which together with their control and auxiliary apparatus are atdiiferent high potentialsrelative to the earth. Each group consists of adoubleanode discharge vessel operating in opposed phases, the anodes 3,3" being supplied from the associated transformer secondary windings 2,2". influence the arcs which burn to the cathodes 5', 5. The cathodeswhich are conductively connected tothe frameworks i, I respectivelywhich support all the apparatus belonging to one particular stage, andthus determine the potential of these frameworks relative to the earth.Of all the auxiliary apparatus which is screened oif to protect itagainst disturbing voltages, only the control circuits for the grids 4,4 are indicated in the drawing. Control voltages are applied to thesegrids over reslstances 6, 6' and grid condensers 1, I. As regards thepresent invention, it is immaterial what form the control voltages have.Generally, however, rectangular impulses with a variable phase and assteep a flank as possible are employed. It is therefore important thatthe device for transmitting the control impulses from the regulating andprotecting apparatus, which is at earth potential, to the grids shouldnot detriin the figure there mentally aiiect this steepness.

The impulses are transmitted at a high frequency, modulated inaccordance with the impulses, over a screened cable, for instanceconcentric high frequency cables l3, l3, which connect the outputtransformers M, M with the insulating transformers 8, 8'. The secondarywindings are at a high potential and can be adjusted for maximum powerreception by means of the condensers ID. The high frequency power isrectified by means of the rectifiers 9. Condensers H deal with theharmonics whilst the direct current impulses are supplied to thescreened transformers I2, l2 which are constructed in such a manner thatthe impulses are transmitted as far as possible without distortion.

The impulses, which in this case control in opposite phases, areproduced by means of the transmitting tubes l6, IS. The anode directcurrent flows from the positive direct voltage terminal 20 over theoscillating coils I5, l to the anode and from thence to the cathode andover a resistance 2| to the negative terminal 22. Part of theoscillation power is fed back to one of the control grids of the tubesover the grid coils I8, l8 which are inductively coupled with theoscillating coils l5, l5, whereby these tubes can be caused tooscillate. The oscillation frequency depends largely on the naturalfrequency of the oscillation circuit formed by the coil l5 and thecondenser IS. The working point of the tubes is thus determined by thenegative grid bias voltage corresponding to the voltage drop of theanode current in resistance 2|. The high frequency oscillations receivetheir impulse form as a result of their modulation by means of thefurther control grids connected to the terminals l1, H. The highfrequency oscillations can be increased or entirely suppressed accordingto the bias voltage applied to these terminals. It is thus possible bycontrolling these grids to determine the phase, form and length of thehigh frequency control impulses as desired, by making them suit the biasvoltage. The task of controlling the grids of the rectifiers which areat a high potential thus resolves itself into a lowvoltage control ofthe grid voltage of the transmitting tubes, 9, problem which has alreadybeen solved and does not require any further explanation. In order toavoid mutual disturbances from an external source the oscillatorcircuits 24 are also mutually screened and decoupled.

For the invention it is unimportant what arrangement of connections isemployed for generating the high frequency oscillations. Other meanswell known in the high frequency art can also be employed for thispurpose. The invention can also be employed in connection with otherconverter arrangements, for instance with inverters. For reasons ofuniformity the grids of all discharge vessels belonging to one and thesame power transmission plant are influenced in the same manner.

I claim:

1. A grid-controlled electron discharge device comprising a vesselhaving a liquid cathode therein, said vessel and cathode being at a highdirect current potential with respect to ground, at least one controlgrid connected to said cathode through a control grid circuit, andbiasing means for impressing control voltage impulses upon said controlgrid circuit; said biasing means including an electronic generator ofhigh frequency oscillations, said generator having a ground potentialconnection and means upon which low frequency impulses may be impressedto modulate the generated oscillations, the frequency of saidoscillations being in the short wave band and substantially higher thanthe frequencies of disturbing voltage surges resulting from theapplication to said control grid of control voltage pulses of the lowfrequency of modulation of said high frequency oscillations, a circuitat high potential with respect to ground and coupled to said generator,rectifier means in said coupled circuit to demodulate the modulatedshort-wave oscillations, thereby to develop low frequency voltageimpulses, and means for transferring the developed low frequency voltageimpulses to said control grid circuit.

2. A grid-controlled electron discharge device comprising a vesselhaving a liquid cathode therein, said vessel and cathode being at a highdirect current potential with respect to ground, at least one controlgrid connected to said cathode through a control grid circuit, andbiasing means for impressing control voltage impulses upon said controlgrid circuit; said biasing means including an electronic generator ofhigh frequency oscillations, said generator having a ground potentialconnection and means upon which low frequency impulses may be impressedto modulate the generated oscillations, the frequency of saidoscillations being in the short wave band and substantially higher thanthe frequencies of disturbing voltage surges resulting from theapplication to said control grid of control voltage pulses of the lowfrequency modulation of said high frequency oscillations, a transmissioncircuit at high potential with respect to ground, means for impressingsaid modulated oscillations upon said transmission circuit, and meansincluding a demodulator coupling said transmission circuit to saidcontrol grid circuit to impress thereon control voltage impulsescorresponding to the low frequency modulation imposed upon the highfrequency oscillatory output of said generator.

PAUL HUBACHER.

